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kekstrom

737 ngx engine ignition, "rwy remain" and wind forecasts

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I have a few simple (i hope..) questions regarding the 737-800 and I thought I'll summarise them all here, hope someone with more knowledge could help me with one or all of my questions/problems.

 

 

1. Whats the difference between "flt" and "cont" in ignition switchhes. I know that cont should be on to provide continuous ignition during t/o and landing and turbulence and flt to be used in in-flight engine restart but whats the difference in the mechanism? Why can't you use "cont" for in-flight restart? What actually happens in engines when flt is selected?

 

 

 

2.What does the "rwy remain --00ft" do in the takeoff setup page of fmc? Havent found a good answer in previous posts. Something about the position update of the navigation system during t/o roll? When should I use it?

 

 

3.On one of the the pre-flight setup pages theres a "wind dir/speed" line, should I put the average total wind speed and direction of my planned route there? Does it affect the FMC fuel consumption calculations? Is there any need to use the specific "rte data" page to enter winds for various legs if I get the assumed fuel calcultions from this already? Or is the idea of entering the wind values for specific waypoints to get correct vnav behaviour? How about the "descent" page "forecast" page, isnt it the same info all over again?

 

Thanks!

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1. FLT is also used in severe turbulence, CONT used in moderate to heavy precipitation conditions.

FLT gives powerful ignition from both ignitors.

 

CONT – •provides ignition to selected igniters when engine is operating and engine start lever is in IDLE

•in flight - provides ignition to both igniters when N2 is below idle and engine start lever is in IDLE.

 

FLT – provides ignition to both igniters when engine start lever is in IDLE.

 

 

2. If you taking off from taxiway intersecting the middle of the runway, you can enter runway remaining for position update with TOGA activation. However with GPS position update turned on you don't have to fill in this field. It's not used for performance calculations. FCOM 11.40.54.

 

3. If you fly short-haul you can enter average wind, and RTE DATA pages would be filled with that wind. For long flight it's reccomended to enter T/C wind here and then fill in winds on RTE DATA page for cruise waypoints. FMC will use it for calculating fuel and ETA.

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2. If you are not using the full runway length, saying that you enter the runway at an intersection which is not the one at the beginning of the runway, you can enter the remaining runway length in this field. As far as I know, FMC will update take-off performance figures, especially the V1 speed should drop.

 

For 1. and 3., I'll leave this to someone better than me, but for 3, the more input you give your FMC, the more accurate its calculations and forecasts will be.

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And i forgot - CONT is also used for anti-ice operations. And Vspeeds will not be changed with runway remaining field change.

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Hi all! To spend my two cents,

 

2. The wind HDG/SPD entry in the route page should be filled by the average wind aloft value, you can obtain this if you use some weather engine addon. This also fills all the legs waypoints with this value (legs page 2/2).

 

On the other hand the wind HDG/SPD entry on the takeoff ref page 2/2 refers to the wind present at the departure airport and is used for V speeds calculations, careful because entering a value here after setting the takeoff ref. page 1/2 resets the takeoff speeds.

 

To be honest concerning avg winds aloft vs. specific values for waypoints the difference is not that great and shouldn't have much of an impact of fuel calculations and TOC/TOD calculations. At the end of the day wind conditions are very variable and on a trip is well possible to find winds shifting from the original forecast therefore upsetting the whole picture, making an average evaluation somehow more accurate if that makes sense? Better having a semi accurate average calculation than a very specific one that only works if everything stays the same.

Hope it helps, Matteo Capocefalo MED1473

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